Vintage Voltage Adapter

Copyright 1998 R.G. Keen. All rights reserved.

Your vintage guitar amplifier is almost certainly not running on the proper AC line voltage.

Most of the older amps in use were designed at a time when the AC power out of the socket was lower. 115VAC is a common rating on power rating plates, some older ones may carry 110 or 112VAC ratings. Over the years, the nominal AC voltage rating has crept up; 115 was "standard" for a while, then came 117, 120. Now many cities have AC that is most commonly 125 or so. We certainly do here in Austin.

This means that amps designed for 112VAC are running at 10% high AC voltages all the time. AC voltage can run even higher at times when the average load on the power net drops off - like in the early evening in spring and fall when all those air conditioners cycle off.

Obviously, most amps will survive life with higher input voltages - they have and do - but it's not necessarily good for them or you, either from the standpoint of repairs costs or tone.

The continuously high AC line voltage means that all the voltages inside your amp are running about10% high all the time. This steady overvoltage can cause your power filter caps to wear out sooner, make tubes die sooner from the higher filament voltage, in general cook things from the extra heat the parts will dissipate. It can also shift the tonal balance in the amp with the higher B+, as well as changing the tone if the amp has been rebiased in a way that unwittingly compensates for the higher B+.

No problem - I'll just use a variac...

Variacs are one way to correct this, but these things are huge, heavy, and carry the temptation to twiddle the voltage just a - little - bit more to see what it would sound like. I looked up variacs in a Mouser Electronics catalog now. It's capable of supplying 3A of AC, weighs about 18lb, costs $68.00, and doesn't include a box to put it in. You'd have to buy a metal box and switches, fuseholder, outlet, and then wire it together, possibly another $15 plus some labor. The cheapest already-boxed up variac they list is a 5A unit. It's ready to use, but weighs even more and costs $188.00.

Assuming that sounds OK, you're going to pay for and haul around this thing. You set up at a gig, measure the AC line voltage, set the variac to give you 110VAC just like your vintage AC-30 needs, and play a thunderous set. You take a break and while you're away, the bass player takes a gander at your new tone-toy, fiddles with the knob, but sees you coming back and leaps away from it so you won't know he messed with it - and accidentally turns the knob up to the 132V maximum. You don't know he touched it - he gets away scot-free - but when you flick it off standby and start into the new set, the amp begins to sound a little funny and some smoke curls delicately out the back...

Vintage AC to the rescue

A handier and less potentially dangerous way to do this for your amp is to put together an AC line voltage corrector. It's not a panacea for bad tone, but it will let your amp work on the power it was designed for in a quick, easy to set up way and may save a set of tubes or an output transformer.

To do this, look at your amp's power rating plate, usually on the back of the chassis. This will tell you the full-power amperes that the amp is expected to draw. Find a low voltage transformer that is rated for at least that many amperes and a voltage that is the difference between the nominal AC power voltage in your town and the rated voltage on your amp, or at least close.

For example, if your wall sockets put out 125VAC, and your amp is rated at 115VAC, 3 Amperes, pick a 10VAC, 3Ampere transformer (8VAC 3Amperes would probably get you close enough, and is common). Wire this transformer up so that the AC line connects to the primary, and the secondary is connected with one end to the end of the primary that is attached to the "hot" side of the AC line. The output of the thing is taken from the "cold" side of the AC line and the free end of the secondary. The secondary adds to or subtracts from the existing AC line voltage depending on which way round the secondary is connected to the primary.

This either adds or subtracts the 10VAC from 125VAC, making your amp see either 115Vac or 135Vac, depending on how you wire secondary. If you wire it to subtract, the AC power to your amp back to about what it was designed to use.


Phase up to it

As I mentioned, there are two ways to connect the secondary. One way, and the voltage between the free end of the secondary and the "neutral" side of the AC line will be the primary line voltage plus the transformer secondary voltage. The other way round, the voltage will be the primary line voltage minus the transformer secondary voltage, which is what you want. Looking at the schematic, the little dots by the transformer windings indicate phasing. The dots indicate ends of a winding which move positive (and then negative - it's AC!) at the same time.

In the top circuit of the illustration, I've shown a way to connect a common 12VCT filament transformer. The primary of the transformer is connected across the incoming AC line. The dotted end of the winding is connected to the "hot" or "line" side of the power line. This causes a voltage to appear across the secondary. With the two dotted ends connected as shown, when the AC line voltage at the dotted end of the primary moves up with respect to the neutral side, the dotted end of the secondary also moves up compared to the non-dotted end of the secondary.

However, with the dotted end of the secondary connected to the dotted end of the primary, the non-dotted end of the secondary moves down compared to the dotted end of the primary. Therefore, at any time, when the two dotted ends are connected, the voltage from the non-dotted end of the secondary to the non-dotted end of the primary is less than the voltage across the primary by exactly the voltage of the secondary.

In the picture shown, with a 120VAC on the input line coming in and a 6V secondary, the AC voltage between the free end of the secondary and the line neutral is 120Vac - 6Vac or 114Vac. If we used a 12Vac secondary, it would be 120Vac-12Vac or 108Vac.

Got all that dotting down? OK - here's the rub: transformer makers don't mark the dotted ends of the windings on power transformers! You have to figure that out by connecting up the windings and measuring the output. It's correct when you get a lower voltage than you started with (or higher if that's what you wanted to do - I don't advise doing that, though). There are tests you can do with batteries and meters and things, but the cut-and-try is faster and easier. Just be careful - this is the AC line voltage you're playing with here. If you can't connect the thing to do the measurements safely, don't try it!

Most of the wiring can be done with wire nuts, as in house wiring. If you get a fuse holder and AC socket with pigtails attached, it can all be done with wire nuts.

A suitable transformer can be bought at Mouser Electronics for $15 or less. To this you will have to add the cost of a metal housing to put it in, fuse holder, line cord, and AC socket, bringing the total up to perhaps $30. If you work cleanly, you can probably mount this in the bottom of a combo amp.

If you decide to make one of these, be very careful about the safety aspects. You will be working with AC line voltages, which can kill you. Do not try this unless you already know how to do primary AC wiring safely. This note does not tell you how to do that. Do not skimp on the housing, line cord, strain relief for the line cord, fuse holder, or AC socket, and be certain that safety ground is connected to the housing and to the secondary AC socket. Note that ALL of the wiring in this power correction box with the exception of the safety ground is connected to the AC primary power, and so is hazardous. The secondary of the transformer, which would normally be isolated from the power line is also hot by virtue of its connection to the AC line.

If you don't have any idea what the proper precautions are but still want one, have a tech wire one up for you. It's not worth your amp - or your life.

If you're clever, you noticed that you could get a transformer with a couple or more voltages and switch select the amount of cut, and or boost. A multitap secondary transformer can give you several steps of lower or higher voltages as shown in the second circuit. With one end of the centertapped secondary connected to the dotted side of the AC line, you have a choice of full line voltage, line voltage -6V, or line voltage minus 12V. You could obviously connect the center tap to the dotted end of the primary and have line voltage, line voltage +6V and line voltage -6V.